,- Bundesanstalt für Geowissenschaften und Rohstoffe, Charakterisierung von Speicher- und Barrieregesteinen, Hannover, Germany (sibylle.mayr@bgr.de)
Since primary and especially secondary creep is the dominating deformation mechanism of rock salt, the latter one is used as direct indicator for evaluation of possible sites for a repository for heat generating nuclear waste in Germany. For determination of long-term stability of these repositories, the creep of salt over long periods have to be well known. Next to the mechanisms described in constitutive laws, corresponding parameters are to be determined. At higher differential stress Δσ = σmax - σmin, the dislocation creep (DC), e.g. Hunsche and Hampel (1999), is effective. A power law can describe it, and deformation rate dε/dt is easy measurable in laboratory. The rate dε/dt is proportional to Δσn , with n=5-7. The exponent is depending on rock salt type and e.g. content of anhydrite and other minerals. Numerous measurements to calibrate the constitutive laws for description of the DC are available. For low differential stress, the pressure solution creep (PSC), e.g. Urai and Spiers (2007), is the dominating process. The deformation rate dε/dt is proportional to differential stress Δσ. Next to e.g. temperature, it additionally depends on grain size. As the deformation rate is very small, only a few measurements (e.g. Berest et al. 2019, 2023; Blanco-Martín et al., 2024) at extremely low differential stress are available to verify and calibrate the constitutive law.
At BGR various data is available for the analysis of possibilities and challenges during the determination of creep rates. On one hand, numerous measurements on e.g. salt from Waste Isolation Pilot Plant (WIPP) and the Morsleben repository site (ERAM) are available, which can be used for calibration of the DC. Although the salt from WIPP-site is bedded salt and from ERAM-site is domal salt, the measurement results do not differ significantly. On the other hand a few measurements on ERAM salt at differential stresses and low temperature (DΔσ = 5 MPa at T = 21°C & and 3 MPa at T= 25°C) are available. They had a duration of up to 40 months and thus give insights into the development of creep over a long period of time.
Processes at small differential stresses dominate the long-term behavior of a final repository. Further measurements at low stresses and simultaneous analyses, e.g. of moisture and microstructure, are therefore necessary as a basis for well-founded long-term safety analyses at different locations.
References
Bérest, P., H. Gharbi, L. Blanko-Martín, et al. 2023. Rock Mechanics and Rock Engineering.
Bérest, P., H. Gharbi, B. Brouard, et al. 2019. Rock mechanics and rock engineering
Blanko-Martín, L., A. Rouabhi, F. Hadj-Hassen, et al. 2024. Rock mechanics and rock engineering
Hunsche, U., and A. Hampel. 1999. Engineering geology
Urai, J. L., and C. J. Spiers. 2007. Paper read at 6th Conference on the Mechanical Behavior of Salt at Hannover.
How to cite: Mayr, S., Gräsle, W., and Zemke, K.: Secondary creep of rock salt: mechanisms and challenges in determination, Third interdisciplinary research symposium on the safety of nuclear disposal practices, Berlin, Germany, 17–19 Sep 2025, safeND2025-116, https://doi.org/10.5194/safend2025-116, 2025.
